The present invention relates to an improved magnetoresistive element and a method of forming the same, and more particularly to a novel ferromagnetic tunnel junction multilayer structure in a magnetoresistive element and a method of forming a tunnel barrier layer in the ferromagnetic tunnel junction multilayer structure.
The ferromagnetic tunnel junction structure has a tunnel barrier layer sandwiched between first and second ferromagnetic layers, wherein the tunnel barrier layer comprises a thin insulation layer having a thickness of a few nanometers. A constant current is applied between the first and second ferromagnetic layers through the tunnel barrier layer, during which an external magnetic field is applied in a direction parallel to the ferromagnetic tunnel unction interface so as to cause a magnetoresistance effect phenomenon, wherein a resistance is varied depending upon a relative angle of both magnetization directions of the first and second ferromagnetic layers. If the both magnetization directions of the first and second ferromagnetic layers are parallel to each other, then the resistance is minimum. If the both magnetization directions of the first and second ferromagnetic layers are anti-parallel to each other, then the resistance is maximum. A difference in coercive force between the first and second ferromagnetic layers causes variations in relative angle in the range from parallel to anti-parallel of the both magnetization directions of the first and second ferromagnetic layers in accordance with the intensity of the external magnetic field. This permits a detection of variations in intensity of the external magnetic field by detecting the variation in resistance. The sensitivity of the magnetic field depends upon a magnetoresistance ratio which is defined by 2P1P2/(1-P1P2), where P1 and P2 are polarization of the first and second ferromagnetic layers. As the polarization of each of the first and second ferromagnetic layers is large, the magnetoresistance ratio is large.
In recent years, the quality of the tunnel barrier layer has been improved, whereby a ferromagnetic tunnel junction exhibiting a high magnetoresistance ratio of about 20% which is near the theoretical value could be obtained. This is disclosed in Journal of Applied Physics, vol. 79, pp. 4724-4729, 1996. This conventional ferromagnetic tunnel junction will be described as follows. FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional ferromagnetic tunnel junction structure formed on a substrate. A CoFe ferromagnetic layer 122 is provided on a substrate 121. An alumina tunnel barrier layer 123 is provided on the CoFe ferromagnetic layer 122. A Co ferromagnetic layer 124 is provided on the alumina tunnel barrier layer 123, so that the alumina tunnel barrier layer 123 is sandwiched between the CoFe ferromagnetic layer 122 and the Co ferromagnetic layer 124.
FIGS. 2A through 2D are fragmentary cross sectional elevation views illustrative of a process of forming the above conventional ferromagnetic tunnel junction structure.
With reference to FIG. 2A, a CoFe ferromagnetic layer 132 is selectively evaporated on a glass substrate 131 in a vacuum by use of a first mask.
With reference to FIG. 2B, an aluminum layer 133 having a thickness in the range of 1.2-2.0 nanometers is selectively evaporated on the CoFe ferromagnetic layer 132 and the glass substrate 131 in a vacuum by use of a first mask.
With reference to FIG. 2C, the aluminum layer 133 is exposed to an oxygen glow discharge to form an alumina tunnel barrier layer 134.
With reference to FIG. 2D, a Co ferromagnetic layer 135 is grown so that the Co ferromagnetic layer 135 has a longitudinal direction perpendicular to a longitudinal direction of the CoFe ferromagnetic layer 132, thereby forming a cross-shaped electrode ferromagnetic tunnel junction device.
The maximum magnetoresistance ratio is 18%.
Other conventional ferromagnetic tunnel junction devices are disclosed in Japanese laid-open patent publication Nos. 5-63254, 6-244477, 8-70148, 8-70149 and 8-316548 as well as 1997 Japan applied magnetics vol. 21, pp. 493-496. The structures of the other conventional ferromagnetic tunnel junction devices are the same as described above. Further, the fabrication process of the other conventional ferromagnetic tunnel junction devices are different from the above described fabrication processes only in that The alumina tunnel barrier layer is formed by exposing an aluminum layer to an atmosphere.
In the meantime, a spin valve structure has been known as being applicable to the magnetic head. The spin valve structure has an electrically conductive and non-magnetic layer sandwiched between first and second ferromagnetic layers, wherein an anti-ferromagnetic layer is provided in contact with the first ferromagnetic layer so that the first ferromagnetic layer serves as a pinned layer whilst the second ferromagnetic layer serves as a free layer. A magnetization direction of the pinned layer is set vertical to a surface of the magnetic medium whilst a magnetization direction of the free layer is set parallel to the surface of the magnetic medium. A leaked magnetic field from the magnetic medium causes change in magnetization direction of the free layer, whilst the magnetization direction of the pinned layer is pinned or remains unchanged, for which reason the leaked magnetic field from the magnetic medium causes change in relative angle between the magnetization directions of the free layer and the pinned layer. The change in relative angle between the magnetization directions of the free layer and the pinned layer causes a variation in resistance of the device. The variation in intensity of the leaked magnetic field can be detected by detecting the variation in resistance of the device.
It has been known in the art to which the present invention pertains that free layer is defined to be a layer which magnetization direction is likely to be changed by an applied external magnetic field as compared to the pinned layer, whilst the pinned layer is defined to be a layer which magnetization direction is unlikely to be changed by the applied external magnetic field as compared to the free layer. If, for example, no anti-ferromagnetic layer is provided adjacent to the pinned layer, then the ferromagnetic tunnel junction devices does not utilize the exchange-coupling field. In this case, a layer having a smaller coercive force serves as the free layer, whilst than another layer having a larger coercive force serves as the pinned layer.
The above tunnel barrier layer of the ferromagnetic tunnel junction device is electrically insulative, so that the current flows across the tunnel barrier layer or in the vertical direction of the junction interface. On the other hand, the intermediate layer between the pinned layer and the free layer of the spin valve structure is electrically conductive, so that the current flows along the junction interface or in the parallel direction to the junction interface. The ferromagnetic tunnel function device and the spin valve structure are common in utilizing the exchange-coupling magnetic field.
In order to apply the magnetoresistive element to the magnetic head, a highly sensitive and stable detection of the leaked magnetic field from the magnetic medium. The first and second ferromagnetic layers of the conventional ferromagnetic tunnel junction devices comprise single layered structures. In order to obtain a large magnetoresistance ratio for high sensitivity, it is required that the first and second ferromagnetic layers have large polarizations are generally large, for example, several tends Oe. For the reason, if the ferromagnetic tunnel junction device has a similar structure to the spin valve structure in order to utilize the exchange-coupling magnetic field, then a remarkable hysteresis characteristic appears on the magnetoresistance curve. This means it difficult to conduct a stable magnetic signal detection.
In order to apply the magnetoresistive element to the magnetic heads, it is required to reduce a resistance of the tunnel barrier layer for reduction of the thermal noise. It was difficult for the conventional fabrication method to reduce a resistance of the tunnel barrier layer.
A signal output voltage level is the key for high density magnetic head. It was also difficult for the conventional technique to obtain a sufficiently high current density and to reduce the resistance without deterioration in characteristics of the device.
It was also difficult for the conventional technique to suppress variations in characteristics of the devices in wafer or between lots for sufficiently high yield.
It may be considered that the above difficulties are caused by the conventional method of forming the tunnel barrier layer. If the oxygen glow discharge is used, then active oxygen as ion or radical is used for oxidation of the conductive layer, for which reason it is difficult to control a thickness of the thin oxide tunnel barrier layer. Since the resistance of the tunnel barrier layer depends upon the thickness of the thin oxide tunnel barrier layer, the difficulty to control a thickness of the tin oxide tunnel barrier layer means it difficult to control the resistance of the tunnel barrier layer. Further, activated impurity gases causes contamination of the tunnel barrier layer, thereby deterioration in quality of the tunnel barrier layer. If the exposure of the conductive layer to the atmosphere is carried out, then dusts in the atmosphere causes formation of pin holes in the tunnel barrier layer. The tunnel barrier layer may also be contaminated with moisture, carbon oxide and nitrogen oxide.